Broadband electro-optic waveguide modulators with low power consumption are necessary for optical communications and high speed signal processing. Compound semi-conductors (e.g. gallium arsenide (GaAs) or indium phosphide (InP)) provide suitable substrates for such communication components as they permit combining both optical and electronic devices using a single substrate. However, microwave loss and dispersion, as well as, inherent differences in phase and group velocites between the optical and microwave signals in a travelling wave modulator limit the available bandwidth.
Two electrode configurations for GaAs modulators are known. These include microstrip configurations using a p-i-n structure and coplanar strip electrode configurations using undoped epitaxial layers grown on a semi-insulating GaAs substrate.
The overlap between the optical and microwave fields in the p-i-n modulators is close to perfect, however the use of n.sup.+ GaAs substrates in the p-i-n causes high microwave losses and dispersion which as a result limit the achievable bandwidth.
Modulators having coplanar strip electrodes fabricated on semiinsulating compound semi-conductor substrates (e.g. gallium arsenide (GaAs)) on the other hand, suffer very little microwave loss and dispersion but unfortunately the velocity mismatch between the optical and microwave signals causes the power consumption to increase with increasing frequency.
U.S. Pat. No. 4,340,873 issued Jul. 20, 1982 to Bastida discloses a periodic transmission structure for slow-wave signals constructed by depositing two spaced parallel conducting bands on a semi-insulating semi-conductor substrate such as gallium arsenide (GaAs). These bands are interconnected by spaced transverse conductors (spaced by a distance less than the wavelength of the microwave signals being transmitted) and central longitudinal conducting strip traversing the transverse conductor strips and separated therefrom by a dielectric film interposed therebetween to provide a succession of concentrated capacitances that slow the signal.
U.S. Pat. No. 4,460,880 issued Jul. 17, 1984 provides circuit matching elements primarily for integrated circuits using a layered structure adapted to increase the specific capacitance of the line without decreasing the specific inductance to slow the signal and thus decrease the wavelength of the signals.
U.S. Pat. No. 4,914,407 discloses a slow-wave structure for monolithic microwave circuits incorporating spaced parallel waveguides on a semiconductor substrate and using a multi-layered ladder like structure including conducting bands parallel to the waveguides and a plurality of cross-tie like conductors separated therefrom by a dielectric layer to provide space capacitances at each cross-tie element to change the capacitance of the system and thereby slow the signal. This system obviously requires that the conductor strips and cross-tie elements be deposited as separate layers with a dielectric layer deposited therebetween, making the structure costly to produce.